Materials which contain intermetallic phases combine metallic and ceramic properties, such as high thermal conductivity, high melting temperature and in some cases satisfactory ductility, and for this reason are apparently adapted for use in the region between conventional metallic high-temperature materials and ceramics, which are strong at high temperatures, but are brittle.
These considerations are of special interest in connection with gas turbines and internal combustion engines, in which the use of improved materials may permit operation at higher temperatures and, as a result, operation with a higher thermal efficiency, and in the design of chemical plants for processes which involve high temperatures and aggressive materials. This is of far-reaching significance because it improves the utilization of energy.
The previous considerations regarding materials which contain intermetallic phases have preferably been concerned with applications such as gas turbine blades for use at temperatures of at least 1100.degree. C. For this reason, mainly compounds having a high melting point have been taken into account, such as TiAl having a melting point of 1460.degree. C. and NiAl having a melting point of 1638.degree. C. However the components of reciprocating internal combustion engines are heated only to much lower temperatures, which presently amount to about 300.degree. C. at the piston head and which, owing to various boundary conditions, cannot be increased as highly as may be desired. On the other hand, a temperature rise by 100.degree. to 200.degree. C. at portions which are under particularly heavy loads would constitute considerable progress. Whereas ceramic materials may be used for that purpose, they will undesirably add to the weight and can be shaped only at a considerable expenditure and can be manufactured only at high cost.
The intermetallic phase alloy Mg.sub.2 Si in accordance wtih DE 37 02 721 A has a higher high-temperature strength than conventional light alloy materials and is relatively light in weight and can well be shaped and easily be produced. That alloy has a melting point of 1092.degree. C., a density of 1.95 g/cm.sup.3 and a virtually negligible homogeneity.
Because Mg.sub.2 Si has a high hardness of VHN 450 at room temperature and VHN 180 at 360.degree. C., a low coefficient of expansion amounting to 7.times.10.sup.-6 K.sup.-1 at room temperature and to 12.times.10.sup.-6 K.sup.-1 at 360.degree. C., and a high resistance to corrosion by hot gas, that material is excellently suited for use in the manufacture of components which are to be subjected to high thermal and mechanical loads in internal combustion engines and particularly for use in the manufacture of components, particularly pistons, for lining the combustion chamber of internal combustion engines. Mg.sub.2 Si has a compressive strength of 1600 mPa at room temperature.
To reduce the brittleness of shaped bodies made of Mg.sub.2 Si and to improve their ductility, grain refining is desirable, which may be effected by addition of up to 42% by weight aluminum and/or up to 22% by weight silicon.
A preferred composition of the Mg.sub.2 Si alloy is represented by a ternary system aluminum-magnesium-silicon in the area which is defined by the eutectic valley, by the quasibinary section, and by 42% by weight. The ductility can also be improved by replacing the silicon by 0.1 to 10% by weight of one or more of the elements germanium, tin, lead or by elements having similar physical-chemical properties.
A fine-grained structure can be achieved by addition of crystallization-promoting agents, such as boron, titanium, lithium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten, individually or in combination.
The hardness of Mg.sub.2 Si can be increased by addition of nickel, copper and/or cerium.
In the production of Mg.sub.2 Si alloys by fusion metallurgy, conventional crucible materials and an inert atmosphere are employed and the molten material is superheated by 20.degree. to 50.degree. C. The material for the permanent molds may particularly consist of iron or copper.
The Mg.sub.2 Si alloys thus produced have a dendritic solidification structure consisting of Mg.sub.2 Si crystallites having an average grain diameter not in excess of about 200 .mu.m. Besides, heterogeneous Mg.sub.2 Si alloys in combination with light metals, such as aluminum and magnesium, contain said crystallites in a distinctly inhomogeneous distribution in the aluminum or magnesium matrix. Owing to the high solubility of gases, particularly hydrogen, in the components of such alloys, the hypereutectic concentrations cannot easily be achieved. Besides, such Mg.sub.2 Si alloys in spite of cooling at a high rate in excess of 10.sup.4 K.times.s.sup.-1 will have an excessively high gas porosity if they contain more than 30 mole percent Mg.sub.2 Si.